Truly quiescent and long-lived stem cells have been postulated but to date not identified in the gut. Doublecortin-like kinase 1 (Dclk1) was proposed as a stem cell marker in the intestine but was also found to be expressed in intestinal tuft cells. Recently, our group has generated Dclk1- Cre-ERT BAC transgenic mice, and we have shown unequivocally that Dclk1+ tuft cells are long-lived, quiescent stem cells that originate from the neural crest, and that Dclk1 also labels extraganglionic progenitors of the enteric nervous system. Our hypothesis is that Dclk1+ stem cells play a role both as neural stem cells and also as reserve intestinal stem cells, which together form a niche for other stem cell populations in the intestine, primarily through generation of the ENS. We will explore this hypothesis through four specific aims. (1) What is the relationship between Dclk1+ stem cells and Bmi1+ intestinal stem cells? We will utilize Bmi1/GFP/+ knockin mice and Lgr5/DTR knockin mice in combination with Dclk1 transgenic lines to determine whether there is overlap between Dclk1 and Bmi1. (2) Does ablation of Dclk1+ progenitors inhibit normal intestinal epithelial homeostasis and the response to radiation injury? Dclk1-Cre-ERT mice crossed to DTR F/F mice will be treated with diptheria toxin and the regenerative response to radiation assessed. (3). Does activation of Dclk1+ progenitors result in intestinal proliferation and increased regeneration following radiation injury? Dclk1 progenitors will be activated by conditional deletion of PTEN or activation of IC-Notch1, followed by analysis of the response to radiation injury. (4). Does ablation of Dclk1 progenitors inhibit in vitro ISC growth, and does activation of Dclk1+ progenitors accelerate in vitro ISC growth. We will use whole intestinal cultures derived from the Dclk1 crosses above to explore the effect of Dclk1 ablation or activation on in vitro growth. Overall, these studies should provide new insights into the role of Dclk1 progenitors as both niche and stem cells in the intestine, and their contribution to intestinal regeneration.